Cyclopentadienyl transition metal compounds are of particular interest in the polyolefin industry today for their use as polymerization catalysts. For example both biscyclopentadienyl and monocyclopentadienyl transition metal compounds (particularly of groups 4, 5 and 6) are known to polymerize olefins when used in combination with an activator, such as an alumoxane or a non-coordinating anion. Likewise U.S. Pat. No. 5,527,752 discloses a new class of olefin polymerization catalysts based on complexes of a transition metal having pi bonded ligands and heteroallyl moieties combined with an activator such as an alumoxane or a non-coordinating anion. Further copending patent application 09/103620 filed Jun. 23, 1998 assigned to Union Carbide discloses new heteroatom containing catalyst systems. Some of the above catalyst systems generate excess hydrogen during polymerization which can limit the molecular weight of the product while others of these catalyst systems must have hydrogen added to control molecular weight and other factors. Indeed, WO/9425497 notes that "[m]etallocene catalysts that are excessively hydrogen sensitive or generate hydrogen may restrict the process to the making of high melt index materials." Hydrogen generating catalyst systems could produce high molecular weight products if the hydrogen level in the polymerization could be controlled. Thus there is a need in the art to provide a method by which higher molecular weight products can be produced from catalysts that generate hydrogen.
In the past many methods have been tried to reduced or eliminate hydrogen production by such hydrogen producing catalysts. For example, hydrogenation catalysts have been used as hydrogen scrubbers to remove hydrogen from traditional Ziegler-Natta catalyst systems. For more details on hydrogenation catalysts and other hydrogen removal systems please see U.S. Pat. Nos. 3,146,223; 4,200,714; 4,851,488; 5,134,208; 5,194,529; and 5,276,115 and EP 0 778 293 A2. Many of the hydrogen removal systems however are expensive and can leave ash in the product that must be removed. Other means of avoiding the hydrogen problem have included avoiding hydrogen generation all together by discovering catalysts that produce high molecular weight polymers without generating significant amounts of hydrogen. For example W096/00246 discloses that certain biscyclopentadienyl metallocenes having a specific kind of substitution on the cyclopentadienyl rings produce high molecular weight polymer without the hydrogen problem. The use of certain biscyclopentadienyl compounds that do not generate significant amounts of hydrogen does not, however, address the issue of how to obtain high molecular weight products from catalysts that do generate hydrogen.
This invention addresses this need by discovering that one catalyst system may be used to modify the polymer produced by another catalyst system. This is unusual in that when a second catalyst system is added to a polymerization system one expects that the second catalyst will simply produce its own polymer and not affect the polymer produced by the first catalyst.
Dual catalyst systems have been used in the past for a variety of reasons. For example WO 98/02247 discloses a dual catalyst system of a metallocene and a non-metallocene (TiCl.sub.4 +alcohol) treated with the contact product of dialkylmagnesium and trialkylsilanol. WO 98/02247 discloses dual metallocene systems and describes the idea that the two different transition metal sources exhibit a different hydrogen response under the same polymerization and hydrogen conditions as critical. Hydrogen response is the sensitivity of the catalyst to manipulation by adding or subtracting hydrogen to the polymerization system to produce different products. Likewise, U.S. Pat. No. 4,935,474 discloses olefin polymerization in the presence of two or more metallocenes (activated with alumoxane) each having a different propagation and termination rate constants. U.S. Pat. No. 5,464,905 discloses a molding polymer composition which comprises a copolymer blend produced from a copolymer produced from two different metallocenes combined with alumoxane and a second copolymer produced with a metallocene and alumoxane. Hydrogen was added to the gas phase polymerizations in the examples. Liquid mixtures of many classes of catalysts are disclosed for use in gas phase polymerization in U.S. Pat. No. 5,693,727. U.S. Pat. No. '727 discloses that more than one liquid metallocene may be employed. The definition of metallocenes includes cyclopentadienyl transition metal compounds and pivalates. However selecting a hydrogen generating catalyst to pair with a hydrogen consuming catalyst is not disclosed. Similarly, EP 0 770 629 A2 discloses a process to produce bimodal polymers using two reactors in series. In some circumstances only the reaction conditions and monomer feeds are changed in the second reactor. In other circumstances a second different catalyst is added to the second reactor. Both cyclopentadienyl transition metal compounds and pivalates are disclosed as catalyst options. However selecting a catalyst having a higher hydrogen index to pair with a catalyst having a lower hydrogen index or selecting a hydrogen generating catalyst to pair with a hydrogen consuming catalyst is not disclosed.